1. Field of the Invention
The present invention relates to a method for producing a toner that is applicable for a developer for developing an electrostatic image in electrophotography, and a toner produced by such the method.
2. Description of the Related Art
Conventionally, as a method for producing an electrophotographic toner used for copiers, printers, facsimiles or complex machines thereof on the basis of an electrophotographic recording method, only a pulverization method had been used. However, recently, a so-called polymerization method in which a toner is formed in an aqueous medium is widely used, and the polymerization method is more commonly used than the pulverization method (Japanese Patent Application Laid-Open (JP-A) No. 07-152202). The toner produced by the polymerization method is called “polymerized toner” or in some countries “chemical toner”, and the polymerization method also include a production method including a polymerization process for convenience. Examples of such polymerization methods in practical use include a suspension polymerization method, an emulsion polymerization method, a polymer suspension (polymer aggregation) method, and ester elongation method.
The polymerization method has an advantage of obtaining a toner having a small particle diameter with ease, a sharp particle size distribution and a substantially spherical shape, compared to the pulverization method. On the other hand, it also has a disadvantage of poor deliquoring efficiency because toner particles are generally deliquored in an aqueous solvent and the polymerization process needs long time. Moreover, after toner particles are solidified and separated from the solvent, the toner particles need to be repeatedly washed and dried. Therefore, the process needs a long time, and a large amount of water and energy.
So-called a spray-dry method, that is a method such that a liquid in which a material is dissolved or dispersed in an organic solvent is jetted from one spray nozzle (spray pore) and does not require the atomization in the aqueous medium, has been performed for a long time (for example, see JP-A No. 57-201248). However, in the spray-dry method, the classification of the formed particles is required as the particle size distribution of the formed toner is broad, and as a result of the classification, the yield is very low.
As a method for solving the problem above, there has recently been proposed a method for forming a plurality of droplets from an orifice having a plurality of pores (nozzles) by applying a pressure pulse from a piezoelectric element (see Japanese Patent (JP-B) Nos. 3786034, and 3786035). As a modified version of this method, the present applicant has proposed a method in which droplets are ejected by vibrating a nozzle (see JP-A No. 2006-293320). Any of these methods (referred as “jet atomizing method” hereinafter) has characteristics to provide particles having a uniform particle diameter, as a plurality of pores (nozzles) are provided and droplets are ejected from each pore (nozzle) one by one.
A fixing device equipped in the general electrophotographic image forming device has a fixing member consisted of a roller or belt which are heated at high temperature, and a cleaning member. When the toner is pressed by the heated fixing member, the wax dispersed in the toner is fused and extruded from the toner to thereby present between the fixing member and the toner. As a result, the adhesion of the toner to the fixing member is reduced, and thus the toner adheres to a recording medium without adhering to the fixing member. This is so called an oilless fixing toner system, and has been a mainstream of the toner system (see JP-A No. 2003-248339, and JP-B No. 3874082). Accordingly, it is common for the raw material of the toner to contain a wax component, and the wax enables to be extruded into a space between the fixing member and the toner at the time of the fixing, by selecting the wax having no solubility to the binder resin.
In the jet atomizing method, which is a subject of the present invention, a liquid in which a toner material is dissolved or dispersed in a solvent (referred as a liquid toner composition hereinafter) is ejected from a nozzle having an extremely small diameter. Here, the binder resin of the toner material is dissolved in the solvent, but the components having different solubility to that of the binder resin, such as a pigment, wax, and a charge controlling agent, are present in the dispersed state which is small enough to the diameter of the nozzle. There is no problem for the liquid toner composition having such dispersed state. However, in the case where the liquid toner composition is left standing, and the dispersion is retained at one place due to a maintenance of a device, the dispersed raw material is slightly aggregated. The aggregated raw material sometimes has a size bigger than the diameter of the nozzle, causing the clogging of the nozzle. The clogging of the nozzle means that the ejection cannot be performed, and it is technically and operationally difficult to remove the clogged material. Therefore, the occurrence of the nozzle clogging is a very serious problem for this production method.
The jet atomizing method, which is a subject of the present invention, does not any restriction for a liquid vibrating system for vibrating the liquid toner composition and a film vibrating system for vibrating the nozzle film. In any of the systems, the method essentially contains a step for applying vibration to the liquid toner composition. When the excess vibration is applied to the liquid toner composition, the cavitation of the liquid toner composition occurs, the vibrations of the toner dispersion in the jetting device and the vibrating film are significantly disturbed, and then the vibration cannot be controlled. As a result, the production efficiency is significantly lowered. Therefore, it is suggested that a degassing step be generally performed after forming the toner solution (see JP-A No. 2006-077166). However, this effect is lowered when the liquid toner composition is left standing, and thus this is not sufficient solution.